The present invention relates to thermal insulation panels and, in particular, to vacuum-based insulating systems.
The advantages of vacuum conditions for providing thermal insulation are well known. However, vacuum conditions are very hard to maintain in many insulating structures. Stainless steel has been employed in the past for construction of insulation panels. In particular, vacuum thermal insulation panels employing two mil thick stainless steel sheets between which glass fiber matting is placed have been disclosed in the past. However, the high cost of stainless steel, together with the problem of forming leak-tight edge seals using stainless steel, has generally discouraged the manufacture of stainless steel insulation panels.
It should be particularly noted that the insulation panels contemplated herein are typically approximately 3 feet square and about 1/2" thick. It is soon appreciated that, at an atmospheric pressure of 14.7 pounds per square inch, pressure loading on the walls of an evacuated insulation panel requires the employment of insulating filler material acting to support the panel walls against collapse due to atmospheric pressure. In such stainless steel panels, it is seen that the stainless steel walls function primarily to preserve interior vacuum conditions. The integrity of the panel is provided by the interior insulating material.
The vacuum level for vacuum insulation is not very stringent. In fact, an interior pressure of as high as 0.1 Torr (100 microns) may be employed satisfactorily. Accordingly, efforts have been made to employ plastic materials for the panel walls in place of the stainless steel walls. However, most gases permeate plastic. The permeation rate depends upon the specific gas and specific plastic. Most plastics have a higher permeation rate for water vapor than for other gases. Oxygen permeates less rapidly than water, while nitrogent and argon generally permeate at lower rates than oxygen. Since very few gases permeate metals, metals have been used as barrier layers on the plastic to reduce their permeation rates. However, metals are also generally highly thermally conductive, thereby generally negating their desirability for use in vacuum thermal insulation panels. In such panels, a low gas permeation rate is required to insure a sufficiently long life for the vacuum conditions.
For example, in a 3'.times.3'.times.1/2" panel, there is a surface area of approximately 1.7.times.10.sup.4 cm.sup.2. If the permeation rate is distributed uniformly over the surface, and the maximum total permeation rate is 1.7.times.10.sup.-9 Torr liters per second, a maximum permeation rate of 10.sup.-13 Torr liters per second per cm.sup.2 is required from the panel walls to result in an interior pressure of less than 0.1 Torr for a 20 year panel lifetime. However, uncoated plastic films with a thickness of approximately 4 mils typically have permeation rates of about 10.sup.-8 Torr liters per second per cm.sup.2 for air at atmospheric pressure and even higher permeation rates for water vapor.
Various manufacturers and individuals have disclosed gas permeation barriers for use with plastic films. For example, in U.S. Pat. No. 3,993,811, issued to W. F. Walles, there appears to be disclosed a method for applying a barrier layer to plastics so as to reduce the gas permeation rate. Additionally, American Can Company supplies an aluminum foil-plastic laminate consisting of sheets of 0.7 mil aluminum separated by plastic with a plastic exterior variously-described for example by catalog numbers E15484-78-B and E16287-79. While this material exhibits excellent permeation properties, the high thermal conductivity of aluminum significantly increases the rate of thermal loss along the edges, when such a material is employed by itself in a vacuum thermal insulating panel. Additionally, Dow Chemical Company, Inc. supplies plastic material (e.g., 4.5 mil thick polystyrene) treated chemically to provide a barrier for reduction of gas permeation. However, the permeation rates are still too high to provide sufficiently long-lived vacuum panels. Additionally, American Can Company also supplies gas permeation barrier coated plastic films made by evaporating a few hundred angstroms of aluminum onto plastic. However, these plastic materials do not meet the gas permeation requirements associated with long-lived vacuum preservation conditions. Lastly, other gas permeation barrier coated plastics, as supplied by E. I. duPont deNemours, Inc. have been examined, and have been found to exhibit gas permeation rates which are too high to meet long-lived vacuum insulating objectives.
Accordingly, it is seen that while many plastic film materials having low gas permeation characteristics or chemically-treated barriers have been developed, none are entirely suitable by themselves for the construction of thermally-insulating vacuum panels. However, the desirability for such panels is nonetheless still present. It should also be noted that while stainless steel panel walls are effective for achieving the objectives of low gas permeation, it is nonetheless an expensive material and exhibits certain edge sealing problems.